1. Field of the Invention
This invention relates to a magnetoresistive sensor and, in particular, magnetoresistive transducer for reading information signals from a magnetic medium.
2. Description of the Related Art
In recent years, magnetoresistive sensors (MR head) using a magnetic thin film or magnetic multi-layer thin film, have attracted attention in the effort to achieve high densities of magnetic recording in equipment, such as a hard disk drive (HDD).
A ferromagnetic film provides an MR effect called an AMR (anisotropic MR). In AMR, electric resistivity may change according to the angle between the direction of the sensing current in the ferromagnetic film and the direction of the magnetic field. A multi film comprising a sandwich structure of ferromagnetic layer/nonmagnetic layer/ferromagnetic layer, may also provide an MR effect called as GMR (giant MR), for example, a spin valve GMR. In a GMR, electric resistivity may change according to the angle between the magnetizations of the two ferromagnetic layers facing each other through the nonmagnetic layer. Both the AMR and GMR are called MR.
Such a recording/reading head comprises a shield-type MR head as a reading head, and an inductive coil as a recording head. The shield-type MR head comprises magnetic shield layers arranged above and below a magnetoresistive effect film (MR film).
When a shield-type MR head and inductive recording head are used in combination, usually, because an MR head requires a good surface flatness of a substrate and for processing reasons, the shield-type MR head is formed on a substrate and, typically, the inductive recording head is formed in laminated fashion on top of it.
Furthermore, the track width (TR) of a reading head (a shield-type MR head), is typically made narrower than the track width (TW) of the inductive recording head, because of signal to noise (S/N) considerations.
FIG. 15 is a cross-sectional view showing the construction of a conventional magnetic recording/reading head consisting of a combination of a shield-type MR head and an inductive recording head.
An MR film 4 is formed on a lower magnetic shield layer 2 on a substrate 1 with a lower reading magnetic gap layer 3 provided between the MR film 4 and the lower magnetic shield layer 2. The MR film 4 is formed, for example, in a stripe form. A pair of leads 5 is connected to both ends of the stripe. An upper magnetic shield layer 7 is formed on the MR film 4 with an upper reading magnetic gap film 6 provided between this upper magnetic shield layer 7 and MR film 4, thereby constituting a shield-type MR head (A).
The upper magnetic shield layer 7 also serves as a lower recording magnetic pole of an inductive recording head. A recording magnetic gap film 8 is formed on the upper magnetic shield layer 7. A recording coil is formed at the rear, though not shown in the drawings. An upper recording magnetic pole 9 is formed on the recording magnetic gap film 8, thereby constituting an inductive recording head (B).
Typically, since a magnetic field response portion of the MR film 4 is defined by the pair of leads 5, the space between the pair of leads 5 represents the width of the track of the reading head (TR). In this case, the leads 5 that supply sensing current to the MR film 4 may be formed by a lift-off method or an ion milling method.
The width of a portion of the upper recording magnetic pole 9 which faces the lower recording magnetic pole 7 through the recording magnetic gap film 8, is the track width of the recording track (TW).
Higher densities of a magnetic recording/reading head can be achieved by reducing the track width and a gap width. For example, in order to achieve recording densities as high as 3G bpsi, a track width of about 1 .mu.m and gap width of about 0.1 .mu.m are required in a shield type MR head 8 as described above. Also, in order to achieve such a narrow track, the leads 5 must be patterned precisely with a narrow separation. In addition, in order to achieve a narrow gap, electrical insulation between the leads 5 and the upper magnetic shielding layer 7 at the upper reading magnetic gap film 6 must be ensured. Therefore, a shallow forward taper is usually applied at the edges of the leads 5 so that the upper reading magnetic gap film 6 may be easily formed while providing good step coverage.
However, the shallow forward tapered shape of leads 5 involves not only the problem that it is difficult to precisely define the width of the reading track (TR) when a track width is reduced, but also problems arising from the method of forming, such as a lift-off method or an ion milling method. In particular, it is difficult to pattern the leads 5 by a lift-off method so as to realize both a narrow track width and narrow gap width.
The problem is due to a low resolution of the resist employed in a lift-off method. A reverse-tapered resist is often employed in a lift-off method in order to suppress the tendency of the film deposited on the resist to wrap round the side wall of the resist, because it will be easy to lift-off the deposited film from the resist. However, at present, the resolution of reverse-tapered resists is poor. Therefore, it is difficult to form a pattern of under a few microns. Further, such reverse-tapered resist lift-off may cause particle contamination when the resist is dissolved. In the case of a narrow-gap head, this can cause insulation breakdown.
On the other hand, in an ion milling method, when the leads 5 are formed having a shallow forward taper, a resist is employed that has the same taper at its edges as the tapered shape of the leads. The leads are formed by directing an ion beam at an inclined angle. The taper of the resist is transferred to the leads in the process of lead patterning. In this process, it is necessary to bake the resist at a comparatively high temperature in order to taper the edge parts of the resist after forming the resist along the shape of the leads. This baking of the resist at high temperature may cause interface diffusion in, for example, a spin valve GMR film. Such interface diffusion may decrease the degree of resistance variation. Further, since it is necessary to employ an alkaline developer to remove the resist, it may cause corrosion of the MR film.
If the shape of the lead edges is not shallowly tapered, this may result in imperfect insulation between the leads and the shield layer. In addition, the center of a flow of a sensing current will be different according to variations in thickness between the center and the edges of a magnetic field response portion of MR film. Therefore, it results in the direction of magnetization at the center of the magnetic field response portion being different from the direction of magnetization at the edges of the magnetic field response portion. Accordingly, even if the magnetization alignment is optimum at the center of the magnetic field response portion of the MR film, optimum magnetization alignment is lost at the edges of the magnetic field response portion of MR film, i.e., in the vicinity of the leads.
As described above, it is difficult to reduce the track width and the gap width for achieving further improvement in recording density. Also, it is extremely difficult to satisfy both the desired taper shape and desired space between leads with a conventional lift-off method or an ion milling method.
For the above reasons, it was considered to be a difficult challenge to precisely define in a shape corresponding to a reduced-size gap. It was also considered to be a difficult challenge to prevent the center of a flow of a sensing current from being different between the center region and the edge region of the magnetic field response portion.